Array.prototype.myFind = function (fn, _this = null) {
    if(typeof fn !== "function") {
        throw new TypeError("fn必须是一个函数")
    }
    for(let i = 0; i < this.length; i++) {
        if(fn.call(_this, this[i], i, this)) {
            return this[i]
        }
    }
    return undefined
}